Q What is wisteria scale?
A Scales are sap-sucking insects covered in a tough shell-like covering. Wisteria scale (Eulecanium excrescens) is larger than average, and feeds on a range of woody plants
Caption: Wisteria scale is easier to spot once the leaves have fallen
Q How do I recognise it?
A The adult scales are large, up to 10mm across, and dome-shaped. The colour is dark brown-black, with a powdery covering. Immature scales are smaller, flatter and paler.
Q Where does it occur?
A Wisteria scale, which originates in China, was first spotted in London in 2001. Scale insects can’t fly, so probably hitched a lift on imported plants and subsequently spread in air currents and on birds' feet. So far reports have been mostly confined to the London area.
Q What plants does it attack?
A Although it seems to favour wisterias, the scale has also been recorded on almond, apple, cherry, peach, sycamore and walnut.
Q What damage does it do?
A Large infestations of scale can weaken plants by removing significant amounts of sap and dissolved nutrients. Wisteria scale has been implicated in dieback of branches, and could probably kill plants that were not in the best of health. Heavy attacks could reduce the quality and quantity of crops on fruit and nut trees.
Q Could I mistake it for anything else?
A There are many other scale insects, but wisteria scale is larger and more domed, making it fairly distinctive.
Q How serious is it?
A So far there is limited experience of wisteria scale in this country, but there has been concern that it could kill attractive, mature wisterias that have taken many years to grow, and could also be a significant pest of fruit and nut trees.
Q What is its life cycle?
A Scales mature in late spring, which is when they are first likely to be spotted. Females lay eggs beneath the scale, and they hatch in early summer. The young nymphs or crawlers move around for a few days, then settle in one place, and become immobile. They develop scales and mature the following year.
Q How do I control an attack?
A Organic sprays, which can also be used on edibles, are based on fatty acids or plant oils. These are much more effective against the immature crawlers than the adults and you will need to make several applications in late May and June. A quick and effective alternative where the scales are accessible is to scrub them off with soapy water in spring, before they lay their eggs.
Q What should I do with affected plants?
A Once the scales have been removed, watch out for re-infestation. Boost heavily affected plants with a liquid feed when in leaf, and mulch in autumn.